Various approaches have been tried for preparing a recording medium using an organic dye. For example, J. Vac. Sci. Technol., 18 (1), 105-109 (1981) discloses a method to form a recording medium by coating the organic dye on a substrate as it is. A method to form a recording medium wherein the organic dye is incorporated in a polymer such as polystyrene having a Tg of about 80.degree. C., exhibiting plastic state at a temperature higher than the Tg, having a low flow temperature and having a low viscosity at a temperature higher than the flow temperature, and the resulting mixture is coated on a substrate to form a recording layer is known (Japan J. Appl. Phys., 22 (2), 340-343 (1983).
When writing on these optical recording media, there is employed a process in which a laser light corresponding to an absorbing wave length of the dye is radiated thereon, the radiated laser light is absorbed by the dye in the recording layer and converted into heat to heighten a temperature, whereby an irreversible deformation of the recording layer, for example a pit, is formed at the radiated spot.
The above method utilizing the irreversible deformation has advantages from a view point of a stable recording of information; however, it is not suitable for a use of the optical recording medium with multiple repeats of writing and erasing.
In the former method, once the medium is recorded, it is essentially unable to recover the medium to the original state by erasing the record, since the record is performed by forming the pit through sublimation, decomposition or scattering of the dye.
In the latter method, the record is performed by the pit formed through deformation by polymer flow. Therefore, for erasing the once-formed pit, there is required a troublesome process such as (a) a whole track containing the pits is subjected to C.W.-erasing process several times with laser light radiation of a high intensity C.W. (continuous wave) or (b) a whole medium is heated by some other method such as heating in an oven at 100.degree. C.-150.degree. C. for about 1 hr, whereby the polymer flow causes the pits to vanish, and (c) the resulting medium is cooled slowly. In spite of the above troublesome process, it is not always satisfactory.
There was further proposed an erasable recording medium composed of (i) a dye-containing expansion layer which forms a dome-like bump by expansion caused by heating with a spot laser light radiation and (ii) a dye-containing retention layer to sustain this form (cf. U.S. Pat. Nos. 4,719,615 and 4,852,075. When recording on this medium, a laser light radiation at the absorbing wave length (.lambda..sub.1) of the dye incorporated in the expansion layer is applied, forming the bump in the expansion layer and the retention layer keeps the form. When vanishing the thus recorded bump, a laser light radiation at the absorbing wave length (.lambda..sub.2) of the dye incorporated in the retention layer is applied, thereby heating the retention layer higher than its Tg, at which it is difficult for the retention layer to keep the bump form.
According to the method described above, it is surely possible to carry out recording and erasing reversibly; however, it still has following problems: (a) it requires two laser lights of different wave length for recording and erasing, (b) it requires two dyes of different maximum absorption wave length, and (c) a fabrication of the laminate structure of the medium is complicated.
In addition to an organic optical recording medium having the problems mentioned above, there is proposed an erasable optical recording medium utilizing a phase transformation of an inorganic crystal by a thermal mode. Since these media are composed of an inorganic material, a convenient film formation such as coating, casting, etc. cannot be applicable, and a relatively complicated and expensive film formation such as vacuum vapor deposition, sputtering, etc. must be employed.